S 11-B, a line of human B lymphocytes transformed with Epstein-Barr virus, has been studied. The cells are B lymphocytes: 100% stain for membrane immunoglobulin (mIg). About 80% contain IgA, mIgA another 30-40% mIgM. None contain in IgG. Cells were separated by cell sorting and cloned. One line which is mIgA- turns IgA+ after several generations in vitro. We hope to derive cells in the process of "switching" isotypes from these clones.